Mirror-type electron gun



June 1967 N. w. w. SMITH MIRRORTYPE ELECTRON GUN Filed Jan 18, 1965 INVENTOR. NORMAN MMSMIT'H United States Patent 3,327,148 MIRROR-TYPE ELECTRON GUN Norman William White Smith, Reigate, England, as-

signor to North American Philips Company, Inc., New York, N.Y.

Filed Jan. 18, 1965, Ser. No. 426,132 4 Claims. (Cl. 313-82) The present invention relates to an electron gun for use in electron beam tubes of the longitudinal interaction type, for example travelling wave tubes.

One of the sources of noise in electron beams is due to the random fluctuation from time to time in the average velocity of electrons. The mean square fluctuation of the average velocity can be computed from the formula where 1/ is the axial component of the emission velocity, I is the emission current and K is a constant. Furthermore it can be demonstrated that the elimination of the high velocity part of the current distribution will produce a large reduction in the mean square fluctuation. The present invention is concerned with the removal of electrons with large emission velocities from the electron stream by a process of collection upon a repeller electrode. It is also concerned with the manipulation of a fraction of the remaining electrons to form an electron beam with reduced mean square fluctuation in the average velocity.

An object of the invention is to provide a low noise electron gun.

The present invention provides an electron gun for an electron-beam tube, comprising a cathode, an annular active surface thereon, heating means for heating said surface to cause emission of electrons therefrom, a circular anode disposed coaxially with said surface for accelerating emitted electrons, a repeller electrode coaxial with said anode for repelling said accelerated electrons, means for impressing on the repeller electrode a voltage to cause electrons with emission velocity components in the axial direction larger than a predetermined value to be collected by the repeller electrode and electrons with emission velocity components in the axial direction smaller than the said predetermined value to be repelled by the repeller potential or the electron space charge potential minimum, and means for causing some of said repelled electrons to flow past the cathode to form a hollow electron beam.

Preferably the anode comprises a hollow cylinder. Suitably the repeller electrode may be a flat annular disc; in this case the current supply conductors to the cathode may be carried along a central spindle passing through the central aperture in the repeller electrode in an insulating manner. One or more transverse electrodes may be provided in the vicinity of the cathode to control the transverse displacement of the electrons flowing back to the cathode. Suitably these transverse electrodes may be hollow cylinders.

The invention will now be described by way of example with reference to the acocmpanying drawings in which:

FIGURE 1 is a graph and FIGURE 2 is a sectional view of an electron gun.

In FIGURE 1 the average electron emission current distribution is illustrated as a junction of axial electron velocity component, this graph being derived from the formula dI 2 -K11, exp (ow referred to above.

The area under the left-hand portion of the curve, that is to say the area shaded in FIGURE 1 represents electrons having low velocities; the unshaded area represents electrons having high velocities and it is these high-veloc- 3,327,148 Patented June 20, 1967 ity electrons that are removed from the electron stream in accordance with the present invention.

FIGURE 2 shows an electron gun comprising a cathode 1 having an annular emissive surface 2 and a heater 3. The cathode assembly is supported on a central spindle 4 which passes through an aperture 5 in a repeller electrode 6 in the form of a flat annular disc. The space between the cathode 1 and the repeller electrode 6 is partially surrounded by a hollow cylindrical anode 7. The cathode assembly is surrounded by a hollow cylindrical transverse electrode 8 and the hollow gun is immersed in a magnetic field whose direction is parallel to the axis of the electron gun. In some experiments leading up to the present invention the cathode was at a potential of 0 volts, the repeller electrode at a potential of 2 volts, the anode at a potential of 50 volts, the transverse electrode at a potential of 3 volts, and a uniform axial magnetic field of 1,000 oersteds was used.

It is thought that the electron gun operates as follows: The electrons are emitted from the emissive surface 2 of the cathode towards the repeller electrode 6. Electrons having emission velocity components in the axial direction larger than a predetermined value are collected by the repeller electrode 6 while the electrons having axial emission velocity components smaller than this predetermined value are repelled by the repeller electrode or the space charge potential minimum. A fraction of these repelled electrons pass through the annular gap between the cathode 1 and the transverse electrode 8. The hollow electron beam thus formed is then accelerated and is allowed to pass through an appropriate longitudinal interaction structure to a collector, neither of which are shown. In FIGURE 2, A indicates the path of a fast moving electron and B indicates thev path of a slow moving electron.

It is essential that electrons should not be emitted from any surface (such as a surface of the transverse electrode 8) facing or substantially tangential to the ultimate direction of the electron beam.

Although an electron gun has been described as having an annular repeller electrode 6 which is flat, this shape is not essential. It may, for example, be convex as viewed on the cathode. It is important that the anode should provide a strong transverse electric field at the outer edge of the cathode but it need not necessarily be cylindrical although it must be of a shape which is rotationally symmetrical about the gun axis in order to maintain electric field uniformity around the periphery of the cathode.

What I claim is:

1. An electron gun for an electron-beam tube having an axis along which electrons are directed, comprising a cathode having, an annular active surface thereon in a plane perpendicular to said axis, heating means for heating said surface to cause emission of electrons therefrom, a hollow circular anode of greater diameter than said surface disposed coaxially with and spaced from said surface for accelerating emitted electrons, a flat annular disc repeller electrode coaxial with and spaced from said anode for repelling said accelerated electrons, means for impressing on the repeller electrode a voltage to cause electrons with emission velocity components in the axial direction larger than a predetermined value to be collected by the repeller electrode and electrons with emission velocity components in the axial direction smaller than the said predetermined value to be repelled by the repeller potential or the electron space charge potential minimum, and means for causing some of said repelled electrons to flow past the cathode in a direction opposite that of electrons leaving said annular active surface for said repeller electrode to form a hollow electron beam.

2. An electron gun as claimed in claim 1 wherein current supply conductors to the cathode are carried along a central spindle passing through a central aperture in and insulated from the repeller electrode.

3. An electron gun as claimed in claim 1 wherein a transverse electrode is provided in the vicinity of the cathode to control the transverse displacement of the electrons flowing back to the cathode.

4. An electron gun as claimed in claim 3 wherein said transverse electrode is a hollow cylinder.

References Cited UNITED STATES PATENTS 2,632,130 3/1953 Hull 31382 X 3,214,632 10/1965 Harman 313'82 X 3,258,626 6/1966 Kino et a1 313-82 JAMES W. LAWRENCE, Primary Examiner. R. SEGAL, Assistant Examiner. 

1. AN ELECTRON GUN FOR AN ELECTRON-BEAM TUBE HAVING AN AXIS ALONG WHICH ELECTRONS ARE DIRECTED, COMPRISING A CATHODE HAVING, AN ANNULAR ACTIVE SURFACE THEREON IN A PLANE PERPENDICULAR TO SAID AXIS, HEATING MEANS FOR HEATING SAID SURFACE TO CAUSE EMISSION OF ELECTRONS THEREFROM, A HOLLOW CIRCULAR ANODE OF GREATER DIAMETER THAN SAID SURFACE DISPOSED COAXIALLY WITH AND SPACED FROM SAID SURFACE FOR ACCELERATING EMITTED ELECTRONS, A FLAT ANNULAR DISC REPELLER ELECTRODE COAXIAL WITH AND SPACED FROM SAID ANODE FOR REPELLING SAID ACCELERATED ELECTRONS, MEANS FOR IMPRESSING ON THE REPELLER ELECTRODE A VOLTAGE TO CAUSE ELECTRONS WITH EMISSION VELOCITY COMPONENTS IN THE AXIAL DIRECTION LARGER THAN A PREDETERMINED VALUE TO BE COLLECTED BY THE REPELLER ELECTRODE AND ELECTRONS WITH EMISSION VELOCITY COMPONENTS IN THE AXIAL DIRECTION SMALLER THAN THE SAID PREDETERMINED VALUE TO BE REPELLED BY THE REPELLER POTENTIAL OR THE ELECTRON SPACE CHARGE POTENTIAL MINIMUM, AND MEANS FOR CAUSING SOME OF SAID REPELLED ELECTRONS TO FLOW PAST THE CATHODE IN A DIRECTION OPPOSITE THAT OF ELECTRONS LEAVING SAID ANNULAR ACTIVE SURFACE FOR SAID REPELLER ELECTRODE TO FORM A HOLLOW ELECTRON BEAM. 